1. Field of the Invention
The present invention relates generally to electron sources, and more particularly to an electron emission source that is driven into an electron emissive state with the assistance of radiation, such as photon emission from a radiation source.
2. Background and Prior Art
Electron emission sources are generally known in the art, and have applicability in many areas of technology which use electron beams, such as vacuum microelectronics applications, semiconductor device manufacturing, electron beam exposure apparatus, electron microscopes, flat panel display devices, radiation spectrometers and imagers, etc.
Various techniques of generating a source of electron beam emission are known in the art, including thermionic emission, field emission and negative electron affinity emission. The surface of any. conducting solid material presents an energy barrier that binds to the host material. Electrons incident upon the energy barrier at energy levels capable of surmounting the barrier may leave the solid, and thus give rise to an emission current.
Thermionic emission is generally achieved by heating a filament to high temperatures, by which electrons gain enough energy to surmount the energy barrier and leave the heated surface. The emission current density of a thermionic emission electron source is given by
J=AT2 exp(xe2x88x92w/kT)xe2x80x83xe2x80x83(1)
where A is a constant, T is the temperature of the source material in degrees Kelvin, k is the Boltzmann constant, and w is the work function of the source material.
Another method of generating electron emission is by altering the energy barrier to make it very thin, whereby electrons can tunnel through it, or to make it very small whereby electrons do not encounter any appreciable barrier to hinder their emission from the surface of the material.
Field emission cathodes present a different mode of emission from a surface than thermionic emission. A field emission source has a tip fashioned from a conducting material such that the energy barrier is very thin. Because contamination of the field emission tip can inhibit emission, field emission electron sources work best when the tip is exposed only to a vacuum.
Another mode of emission is provided by negative electron affinity devices. In such devices a coating is applied to a surface that produces xe2x80x9cbendsxe2x80x9d in the energy bands such that there is no barrier present to prevent the emission of electrons into a vacuum. Hence negative electron affinity emission also generally is carried out where the treated surface is exposed only to a vacuum.
Regardless of the material, electron emission is best realized where the material has a large density of freely conducting electrons. For this reason, degenerately doped semiconductor materials are often used for both field emission and negative electron affinity devices, because of the high density of freely conducting electrons present in the material. Emission achieved primarily by increasing the voltage drop across the emission region of the device.
There exists a need in the art for improvement in such electron emission devices, and specifically to provide electron emission devices that can use semiconducting material that does not have to be degeneratively doped.
The present invention overcomes the disadvantages discussed above by providing an electron emission device that can be manufactured from a semiconductor material that is very pure (i.e., with a very low doping concentration), or from a semi-insulating material that has a very low doping concentration.
The device according to the present invention uses the change in conductivity caused by interactions of ionizing radiation within the device to increase the population of free electrons near the surface of the device. The ionizing radiation can be provided by any suitable mode of radiation, such as electromagnetic radiation, charged particle radiation, or neutron radiation. The free electrons excited by the ionizing radiation either can drift or diffuse to the emission region, whereby the conductivity in the emission region increases, allowing for increased electron emission from the emission region.
In particular, the present invention provides a radiation assisted electron source, including semiconducting or semi-insulating material capable of producing electron-hole pairs, a source of radiation for providing incident radiation to the material, the incident radiation exciting electron-hole pairs within the material, an emission mechanism formed in the material, and a transport mechanism for driving electrons of the electron-hole pairs to a local region of the emission mechanism, where the electrons are released from the emission mechanism to provide an electron beam.